The present invention relates to brushless motor systems.
Recent years have seen great simplification of DC motors, with corresponding benefits in cost and reliability. Historically most DC motors have used brushes to provide current to the correct phase of the rotor windings, and this persists in motors for consumer products; but for products where reliability and lifetime are needed, electronic commutation is now used. By using semiconductor switches (e.g. FETs) to switch current to the appropriate field winding, the need for replacement of brushes, and the attendant ozone generation, production of conductive dust, and potential for sparking, can be avoided.
Initially electronic commutation was usually accomplished by using some other mechanism to sense the physical position of the rotor. The transducers are typically Hall cells mounted at strategic locations in the motor, in order to provide position information for the commutation circuitry. However, the need for these costly components can be eliminated by obtaining motor position information based on the BEMF of the unenergized (floating) winding.
"BEMF," or back electromotive force, is the voltage induced on a winding, by the changing magnetic field which is present inside the motor, when the winding is not being electrically driven by the external driving circuit. The proximity of a rotor pole contributes to the changes in the magnetic field (due to the magnetic field in the rotor), and therefore the BEMF provides some information about the instantaneous position of the rotor. Even though the magnitude of the BEMF is highly dependent on the specific motor architecture (and possibly also on the load conditions), a change in the sign of the BEMF will occur when a rotor pole passes the center of the floating armature coil. Thus detection of zero-crossings in the BEMF can in principle provide adequate information about rotor position.
Many publications have discussed the problems of brushless DC motor control, including e.g. Pouilloux, "Full-wave sensorless drive ICs for brushless DC motors," 10 ELECTRONIC COMPONENTS & APPLICATIONS 2 (1991); Antognini et al., "Self synchronisation of PM step and brushless motors; a new sensorless approach," in ACTUATOR 90: PROCEEDINGS OF 2ND INTERNATIONAL TECHNOLOGY-TRANSFER CONGRESS at 44 (ed. K.Lenz 1990); Bahlmann, "A full-wave motor drive IC based on the back-EMF sensing principle," 35 IEEE TRANSACTIONS ON CONSUMER ELECTRONICS 415 (1989); Paraskeva et al., "Microprocessor control of a brushless DC motor," in PROCEEDINGS OF THE CONFERENCE ON DRIVES/MOTORS/CONTROLS 84 at 80 (1984); U.S. Pat. No. 5,343,127 of Maiocchi, "Start-up Procedure for a Brushless, Sensorless Motor;" U.S. Pat. No. 5,319,289 of Austin et al., "Adaptive Commutation Delay for Multi-pole Brushless DC Motors;" U.S. Pat. No. 5,202,616 of Peters et al., "Bipolar or Unipolar Drive Back-EMF Commutation Sensing Method;" Hanselman, BRUSHLESS PERMANENT-MAGNET MOTOR DESIGN (1994); and T.J.E. Miller, BRUSHLESS PERMANENT-MAGNET AND RELUCTANCE MOTOR DRIVES (1993); all of which are hereby incorporated by reference.
Background: Power Transistor Control Using Pulse Modulation
When a power transistor is used to drive a load which can draw high current (such as a motor winding), the transistor's power dissipation will be high when it is only partially turned on. Thus in such applications the transistors are typically switched on or off (subject to slew rate constraints), but are not operated for any significant duration in an intermediate state. Therefore, when analog control of output current is required, this is commonly synthesized by switching the transistor with a waveform which is then averaged, by a capacitor of acceptable size, to provide the desired analog current waveform.
One of the most common ways to do this is pulse-width modulation (PWM). In pulse-width modulation the duration of each pulse is adjusted to provide the desired average current level; the pulses themselves may occur at a constant frequency, or may be separated by constant durations.
In motor control systems generally, the control logic gets one or more data inputs to determine the velocity of the motor, and accordingly controls transistor switching to apply the correct drive current to the motor windings. The control relation normally seeks to maintain the motor at a predetermined constant speed (or sometimes at a variable speed determined by a command input). In integrated motor control systems, the command outputs from the control logic are typically provided to a PWM circuit, which provides a pulse train with the desired duty cycle to the gate of a power FET (either directly or through additional buffering stages).
Various other pulse modulation schemes have been proposed, involving introduction of techniques such as burst length modulation or frequency modulation. However, the two types of PWM are extremely simple, and are the predominant technique used for motor control.
In order to establish proper commutation during the operation of a multi-phase brushless motor, it is essential to determine the phase of the motor windings. Normally, two windings are driven from phase-to-phase, making the Back EMF (BEMF) available at the floating phase. This BEMF can then be examined to determine the electrical phase. This information can then be used to determine commutation timing, as well as providing a signal representative of the rotation of the motor (Tachometer signal).
A problem arises when the driving signal from phase to phase is Pulse Width Modulated (PWM). The recirculating clamps necessary for providing the current path in PWM are present also at the floating phase. This restricts the floating phase voltage to the power and ground potentials. However, the BEMF voltages present on the motor phases add up in such a way that, during recirculation, the floating phase is driven above or below the positive or negative rails, depending on whether the recirculation clamps are at the positive or negative rail. This results in current flowing through the floating phase. This current is objectionable for at least two reasons: 1) It produces a torque in the direction opposite rotation (Braking), increasing torque ripple and resulting in acoustic noise, and 2) This current flowing through the phase resistance and inductance, adds to the floating phase BEMF, making it impossible to directly measure the BEMF at the floating phase.
This invention eliminates the current in the floating phase during the recirculation time of the other phases.
Summary of Disclosed Innovations
In order to prevent current flowing in the floating phase of the motor during recirculation, the PWM operation is modified from the conventional high or low side chopping, to alternating high and low side depending on the phase angle of the Back EMF of the floating phase. Thus, the high side is chopped during the time the floating leg's Back EMF is positive, and the low side chopped during the time it is negative. The switch between high-side-chopping and low-side-chopping can be made, for example, at the time of zero-crossing (in the voltage on the floating leg), i.e. halfway through the driving phase, or can also be made at any other time before the current rise in the floating leg occurs.
Thus the disclosed inventions avoid excessive power consumption and acoustic noise generation. One advantage of the present invention is that additional PWM circuits are not needed, since one PWM circuit can simply be switched over from pull-up to pulldown. Another advantage is that the number of switching events is not increased, so the electrical noise radiated is not increased.
This invention requires the ability to select whether the low or high leg will be chopped, and this implies some hardware constraint; but otherwise the present invention is highly compatible with existing structures. For example, only one PWM oscillator is necessary, since this oscillator can simply be switched from operating the high side to operating the low side as required.
Note that this innovative operation is applicable to any brushless DC motor, regardless of whether position sensing is used or not.
In theory, the floating-leg current could also be prevented by chopping the high and low sides simultaneously--but this is difficult to implement. However, the present invention is a much simpler solution for which existing hardware can more easily be adapted.